1. Field of the Invention
The present invention relates to an active matrix type liquid crystal display apparatus (TFT-LCD) using a thin film transistor (TFT) as a switching element and to a manufacturing method therefor and, more particularly, to a method of manufacturing a TFT array substrate, which has a transparent conductive film provided on a reflective pixel electrode, at low cost.
2. Description of the Related Art
The active matrix type liquid crystal display apparatus is configured so that a liquid crystal material is placed between a TFT array substrate, in which thin film transistors are arranged in a matrix manner on a glass substrate, and a color filter substrate having counter electrodes, and that the orientation of the liquid crystal material is controlled by the electrodes formed on the substrates to thereby display an image. The active matrix type liquid crystal display apparatuses have widely been developed as display apparatuses for laptop personal computers and OA monitors.
Ordinary related liquid crystal display apparatuses include a transmissive type liquid crystal display apparatus, which is adapted to display an image by installing a light source on a back surface or a side surface thereof, and a reflective type liquid crystal display apparatus adapted to display an image by installing a reflecting layer therein and by reflecting ambient light on the surface of the reflecting layer. The transmissive type liquid crystal display apparatus has a problem that in a case where ambient light is very bright, a display indicated by displaying-light cannot be observed because the displaying light is dark in comparison with the displaying-light. Also, the reflective type liquid crystal display apparatus has a problem that visibility is extremely degraded in a case where ambient light is dark. Thus, a liquid crystal display apparatus (hereunder referred to as a semi-transmissive type liquid crystal display apparatus), which has a transmissive pixel electrode and a reflective pixel electrode at each single pixel part, has attracted attention.
The structure of a related semi-transmissive type liquid crystal display apparatus is briefly described below.
A thin film transistor formed on a transparent insulating substrate has agate wiring including a gate electrode, a source wiring including a source electrode, a drain electrode formed in the same layer as that in which the source wiring is formed, and a channel that is provided between the source electrode and the drain electrode and that is constituted by a semiconductor film. Additionally, a passivation film and an organic film, which has an uneven surface, are formed therein to cover the thin film transistor. A transmissive pixel electrode, which is constituted by a transparent conductive film such as an ITO film, and a reflective pixel electrode constituted by a film made of a material (a high reflectance material) having a high optical reflectance are disposed on the organic film. These electrodes are connected to the drain electrode through tapered contact holes provided in the passivation film and the organic film. Additionally, such a TFT array substrate can be manufactured by performing six steps of a photolithography process (see, for example, JP-A-2003-248232 (pages 9 to 10, FIG. 3)).
In the TFT of the related semi-transmissive liquid crystal display apparatus, the corrosion of a transparent pixel electrode constituted by a transparent conductive film is caused by a cell reaction. The technique of forming a high melting-point metal film, such as an Mo film, just under the reflective pixel electrode is known as a countermeasure thereagainst (see, for instance, JP-A-11-281993 (FIG. 4)).
Further, in a case where an Al-alloy is used as the material of a reflective pixel electrode in a semi-transmissive liquid crystal display apparatus, as described above, a defective indication, such as a flicker, occurs due to a difference in a work function between the material of the reflective pixel electrode and that, such as ITO, of the transparent conductive film used as that of the color filter substrate serving as the counter substrate. The technique of forming a transparent conductive film on the reflective pixel electrode is known as a countermeasure thereagainst (see, for example, JP-A-2003-255378 (pages 4 to 7, FIG. 2)).
Also, the technique of turning the reflective pixel electrode and the transmissive pixel electrode upside down in the stacked layers is known as a countermeasure against the flicker and as the solution of the defective indication due to cell corrosion (see, for instance, JP-A-2004-46223 (FIGS. 10 and 31)).
A material including Al, which excels in reflectance and in an etching shape, as a principal ingredient is suited to the high-reflectance material of the reflective pixel electrode. However, the following problem occurs in the semi-transmissive liquid crystal display apparatus. That is, although the photolithography process for performing the patterning of the reflective pixel electrode usually has a step of immersing the electrode in developer, the pixel electrodes are corroded by a cell reaction between the alkaline developer and a transmissive pixel electrode, which serves as a foundation and is a transparent conductive film, such as an ITO, at that time.
The technique of forming a high-melting-point metal film, such as a Mo-film, which is described in JP-A-11-281993, is taken as a countermeasure against the cell reaction. This is because electrolyte, such as developer, is prevented by using a Mo-alloy film as a protective metal film from simultaneously being in contact with the Al material of the reflective pixel electrode and the ITO. Thus, occurrence of electrolytic corrosion between the Al material and the ITO can be prevented. In a case where the coverage of the Mo-alley on a pattern edge of the pixel electrode is poor and a crack or the like is present, the simultaneous contact of the developer with both of the ITO and the Al-material may occur due to the crack in the Mo-alloy. Therefore, this technique has problems that the corrosion-resistance of the electrode is not necessarily sufficient, and that there is the necessity for increasing the number of steps of forming films to form the Mo-alley film as the foundation thereby to increase the manufacturing cost of the display device.
Also, in a case where an Al-alloy is used as the material of the reflective pixel electrode in the semi-transmissive type liquid crystal display apparatus, a defective indication, such as a flicker, occurs due to the difference between the Al-alloy and the material of the transparent conductive film, such as ITO, used as the counter electrode on the color filter substrate serving as the counter substrate. Thus, the technique of forming the transparent conductive film on the reflective pixel electrode, as described in JP-A-2003-255378, is known as a countermeasure thereagainst. However, even in this configuration, the reflective pixel electrode and the source electrode are constituted by similar materials, such as Al. Thus, when etching is performed on the reflective pixel electrode, the source electrode is eroded by etchant for the reflective pixel electrode, so that disconnection of the source electrode may occur. Consequently, it is necessary to provide connecting metal layers in the contact hole parts provided in the organic film. Accordingly, this technique has problems that a new mask pattern is needed, that the number of steps increases, and that the manufacturing cost of the display apparatus increases.
Also, the technique of turning the reflective pixel electrode and the transmissive pixel electrode upside down in the stacked layers, as described in JP-A-2004-46223, is known as the countermeasure against the flicker and as the solution of the defective indication due to the cell corrosion. However, in such a configuration, usually, it is necessary for ensuring the coverage of the contact hole part that the thickness of the transparent conductive film, such as an ITO film, is about 50 nm or more. Thus, this technique has a problem that the reflectance of the reflective pixel electrode is degraded.
The invention is accomplished to solve the aforementioned problems. Accordingly, an object of the invention is to provide a manufacturing method enabled to manufacture a liquid crystal display apparatus, which excels in reflectance and prevents occurrences of a flicker and of cell corrosion, without increasing the number of manufacturing steps, thereby to obtain a high-quality low-cost liquid crystal display apparatus.